Gravimetric Analysis

Assignment #5Assignment #5

Announcements

Thursday I will be unavailable for office Thursday I will be unavailable for office hours at 3 pm; I will make myself hours at 3 pm; I will make myself available Thursday 9 – 10 am and available Thursday 9 – 10 am and Thursday 4-6 pm (assuming thesis Thursday 4-6 pm (assuming thesis committee breaks at 4pm)committee breaks at 4pm)

Addition: Friday 9 – 10 am office hourAddition: Friday 9 – 10 am office hour

Gravimetric Analysis- What is It?

Definition:Definition:a a precipitationprecipitation or or volatilization volatilization method based on the determination method based on the determination of weight of a substance of known of weight of a substance of known composition that is chemically related composition that is chemically related to the to the analyteanalyte

analyte - chemical element or analyte - chemical element or compound of interestcompound of interest

Gravimetric Analysis- What is It? Reaction:Reaction:

aA + rR -----> AaA + rR -----> AaaRRr pptr ppt

where:where:– a is # of moles of analyte Aa is # of moles of analyte A– r is # of moles of reagent Rr is # of moles of reagent R

– AAaaRRrr is a pure, insoluble precipitate is a pure, insoluble precipitatewhich we can dry and weigh or ignite which we can dry and weigh or ignite to convert to something we can weighto convert to something we can weigh

– ppt=precipitateppt=precipitate

T.W.Richards

1914 Nobel Prize to T.W.Richards 1914 Nobel Prize to T.W.Richards (Harvard University) for the atomic (Harvard University) for the atomic weights of Ag, Cl, and Nweights of Ag, Cl, and N

Richards and his group Richards and his group determined atomic weights of 55 determined atomic weights of 55 of the 92 known elements using of the 92 known elements using gravimetrygravimetry

T.W.Richards

““Every substance must be assumed to Every substance must be assumed to be impure, every reaction must be be impure, every reaction must be assumed to be incomplete, every assumed to be incomplete, every method of measurement must be method of measurement must be assumed to contain some constant assumed to contain some constant error, until proof to the contrary can be error, until proof to the contrary can be obtained. As little as possible must be obtained. As little as possible must be taken for granted.”taken for granted.”

7 Steps in Gravimetric Analysis

Dry and weigh sampleDry and weigh sample Dissolve sample Dissolve sample Add Add precipitating reagentprecipitating reagent in excess in excess CoagulateCoagulate precipitate usually by heating precipitate usually by heating Filtration-separate ppt from Filtration-separate ppt from mother mother

liquorliquor Wash precipitate (Wash precipitate (peptizationpeptization)) Dry and weigh to Dry and weigh to constant weightconstant weight

Suction Filtration

Filter flaskFilter flask Buchner funnelBuchner funnel Filter paperFilter paper Glass fritGlass frit Filter adapterFilter adapter Heavy-walled rubber Heavy-walled rubber

tubingtubing Water aspiratorWater aspirator

Suction Filtration

Mother liquorMother liquor

Advantages/Disadvantages

Experimentally simple and Experimentally simple and elegantelegant

AccurateAccurate Precise (0.1-0.3 %)Precise (0.1-0.3 %) Macroscopic technique-requires Macroscopic technique-requires

at leastat least 10 mg ppt to collect and 10 mg ppt to collect and weigh properlyweigh properly

Time-consuming (1/2 day?)Time-consuming (1/2 day?)

What Do We Get Out of Gravimetry?

% of analyte, % A% of analyte, % A

%A = %A = weight of analyte weight of analyte x 100 x 100 weight of sample weight of sample

weight of ppt directly obtained -weight of ppt directly obtained ->?A>?A

How Do We Get %A?

% A = % A = weight of ppt x gravimetric factor (G.F.)weight of ppt x gravimetric factor (G.F.) x 100 x 100 weight of sampleweight of sample

G.F. = G.F. = a FW[analyte]a FW[analyte] b FW[precipitate] b FW[precipitate]

G.F. = # gms of analyte per 1 gm G.F. = # gms of analyte per 1 gm pptppt

Gravimetric Factor

X apples + Y sugar = Z apple piesX apples + Y sugar = Z apple pies

What is this relationship in chemistry?What is this relationship in chemistry?

The Gravimetric Factor

G.F. = G.F. = a FW[analyte]a FW[analyte] b FW[precipitate] b FW[precipitate]

AnalyteAnalyte pptppt G.F.G.F.CaOCaO CaCOCaCO33

FeSFeS BaSOBaSO44

UOUO22(NO(NO33))22..6H6H22OO UU33OO88

CrCr22OO33 AgAg22CrOCrO44

Gravimetric Factor

AnalyteAnalyte pptppt G.F.G.F.CaCaOO CaCaCOCO33 CaO/CaCOCaO/CaCO33

FeFeSS BaBaSSOO44 FeS/BaSOFeS/BaSO44

UUOO22(NO(NO33))22 UU33OO88 33UOUO22(NO(NO33))22/U/U33OO88

CrCr22OO33 AgAg22CrCrOO44 CrCr22OO33//22AgAg22CrOCrO44

Naming is critically important (next class)Naming is critically important (next class)

Why AgCl?

Reaction is highly Reaction is highly selective selective - no - no interferentsinterferents– 2AgCl ----> 2Ag + Cl2AgCl ----> 2Ag + Cl22(g)(g)

AgCl is insoluble in water, i.e., only AgCl is insoluble in water, i.e., only slightly soluble in water-losses slightly soluble in water-losses negligiblenegligible– 1.4 mg/L at 201.4 mg/L at 2000C C 22 mg/L at 10022 mg/L at 10000CC

Why AgCl is a Good Precipitate?

Small mass of analyte yields large Small mass of analyte yields large mass of precipitate-mass of precipitate-sensitivesensitive techniquetechnique

AgCl precipitates in curds/lumps that AgCl precipitates in curds/lumps that can be easily collected, dried, and can be easily collected, dried, and weighedweighed

Precipitate (ppt) is not Precipitate (ppt) is not hygroscopichygroscopic

Problem

Consider a 1.0000 g sample containing Consider a 1.0000 g sample containing 75% potassium sulfate (FW 174.25) and 75% potassium sulfate (FW 174.25) and 25% MSO25% MSO44. The sample is dissolved and . The sample is dissolved and

the sulfate is precipated as BaSOthe sulfate is precipated as BaSO44 (FW (FW

233.39). If the BaSO233.39). If the BaSO44 ppt weighs ppt weighs

1.4900, what is the atomic weight of M1.4900, what is the atomic weight of M2+2+ in MSOin MSO44? ?

ANS: MgANS: Mg2+2+

Answer The hard part is setting up the correct The hard part is setting up the correct

equation (good stoichiometry skills are equation (good stoichiometry skills are essential here!):essential here!):

Rearranging and solving:Rearranging and solving:

06.96

39.233*25.0

25.174

39.233*75.04900.1

x

)(12.24;06.96

3475.584855.0

2

Mgxx

Problem

A mixture of mercurous chloride (FW 472.09) A mixture of mercurous chloride (FW 472.09) and mercurous bromide (FW 560.99) weighs and mercurous bromide (FW 560.99) weighs 2.00 g. The mixture is quantitatively reduced 2.00 g. The mixture is quantitatively reduced to mercury metal (At wt 200.59) which weighs to mercury metal (At wt 200.59) which weighs 1.50 g. Calculate the % mercurous chloride 1.50 g. Calculate the % mercurous chloride and mercurous bromide in the original and mercurous bromide in the original mixture.mixture.

ANS: 0.5182 g ANS: 0.5182 g

Answer

Again, important to set up correct Again, important to set up correct equation:equation:

Rearranging and solving:Rearranging and solving:

99.560

259.200*2

09.472

*59.200*250.1

xx

gx

x

5182.0

50.127151.08498.0